I formed a fleet consisting of a medium hull capital custom ship and the mercenary The Ysegard, which is classified as a support ship for some reason. Anyway, I attacked a precursor anomaly, defended by the usual fleet of pirates. My capital ship did all of the attacking until it was destroyed, and only after that did the Ysengard start to take part in the battle. At that point the battle viewer showed negative hit points and ships:
Edit: I have no idea why the screenshot is not showing up clearly. I note that the Stardock forum system seems to have changed the way you embed jpgs and pngs, perhaps that's it. Anyway, you should be able to see that the hitpoints and number of ships in my fleet are negative.
I would have expected the Ysengard to fire at the pirates before my other ship was destroyed. Perhaps it was too far away, but the viewer was showing it in front. Anyway, the negative numbers seem like a definite bug.